My concern as a seminar leader in the Teachers Institute is to explore ways of conveying the knowledge and excitement of science and technology to the Fellows. A "hands-on"approach, using experiment and demonstration, is one way of conveying realistically the nature and methods of science. In these days of scarce resources, however, the Fellows may find it difficult to transfer such experiences to their classrooms. We should therefore seek interesting examples from which we can draw scientific generalizations that are broadly applicable. Such examples can often be found among the technological artifacts in the studentsU own locality. Let me illustrate with one about which I learned last summer Q an artifact that would be very useful in the study of light and radio.
In Marion, Massachusetts, on the western shores of Buzzard's Bay, there existed, from shortly before the First World War until a decade after the Second, a forest of 400-foot-high towers supporting a radio antenna. Here Guglielmo Marconi had constructed one of the Marconi Company's eleven wireless transmitting stations to provide worldwide communications. During the First World War the U. S. Navy appropriated the station for the nationUs defense, and in l9l9 it formed a part of the assets of the new company called the Radio Corporation of American (RCA).
The Marion installation operated at two different wave-lengths: ll,620 and l3,420 meters. The frequencies were therefore very low, about 26,000 and 22,000 hertz (cycles per second) respectively. (The numerical product of wave-length and frequency is the speed of light.) These low frequencies were chosen in order to enable trans-Atlantic transmission. Nature has provided, in the surface of the ocean and the ionosphere above, two facing mirrors, each capable of reflecting electromagnetic waves with minimal loss. A small bit of geometry and geography permits one to find that for trans-Atlantic radio communications (from Marion to LandUs End) there have to be four bounces of the waves, since the ionosphere is about l00 kilometers above the earthUs surface. This layer is composed of a very low density of electrons and ions which itself has a natural frequency of oscillation, the plasma frequency (about a million hertz.) As long as the radio wave frequency is less than this plasma frequency, the ionosphere reflects the radio wave. For frequencies higher than the plasma frequency, the mirror ceases to work but instead lets the radio wave leak out into space.
Another reason for the very low frequencies was the need for high radio frequency (r.f.) power. The first wireless technologies were not very efficient; in particular the receivers were quite sensitive. For Marconi's system the transmitters radiated 200,000 watts of r.f. power Q a level that could only be attained by interrupting high voltage electrical discharges. By opening and closing these spark gaps, the Morse code of dots and dashes could be impressed or modulated upon the waves. What is the visible consequence of operating a station with such low frequencies and long wave-lengths? In order to send radio waves efficiently into the atmosphere, an antenna needs to be a quarter of the wave-length. The elder residents of Marion still speak in awe of the rows of towers from which the mile-long horizontal antenna was arrayed.
The Marion station had its main impact during the twenties, when it competed successfully with the older trans-Atlantic cable. Because its capital costs were much less than those of the undersea cable, it was immediately of economic importance. From it, for example, the Wall Streeters of the day obtained the London prices before the stock trading opened in New York. A number of fortunes were based on this timely information. As the next decade arrived, RCA expanded their services to include voice transmission. By then, vacuum tube electronics had led to more sensitive receivers. These new stations operated at a higher frequency than Marion so that voice frequencies could more easily be impressed upon the radio wave, yet not so high a frequency that the ionosphere would cease to function as a mirror.
What general point is here most relevant to a seminar on light and radio? Simply this: in all instances of electromagnetic reflection, the physics remains the same though the numbers are different. Mirrors only work when electric charges are able to move under the influence of the oscillating electric field associated with the wave. The frequency above which electromagnetic waves are transmitted rather than reflected varies with the square root of the density of free electric charges. Thus the ionosphere, which has a low charge density, is a reflector of long waves; while aluminum metal, which has a very high charge density, can also reflect light, whose wave-length is very small. High frequencies also have a technological advantage. The higher the frequency of the electromagnetic wave, the greater the possible rate of modulation. As a result, more simultaneous channels become available with a single transmitter station, which decreases the cost for each channel. As we have proceeded to use higher frequencies for worldwide communications, we have had to replace natureUs mirrors with active reflectors or re-broadcasting stations placed upon satellites, which are much above the ionosphere. Through all these technological changes, however, the physics remains the same.